public static String findSmallestNumber(String pattern) {
        if (pattern == null || pattern.isEmpty() || !pattern.matches("[MN]+")) {
            return "-1"; // Invalid input handling
        }

        StringBuilder result = new StringBuilder();
        Stack<Integer> stack = new Stack<>();
        int num = 1;

        for (int i = 0; i <= pattern.length(); i++) {
            stack.push(num++);
            if (i == pattern.length() || pattern.charAt(i) == 'N') {
                while (!stack.isEmpty()) {
                    result.append(stack.pop());
                }
            }
        }

        return result.toString();
    }